450 THIRD REPORT— 1833. 



membrane must exist. It cannot be supposed that a mass of 

 the magnitude of the brain, and possessing so definite an or- 

 ganization, should form an exception to tlie fabric of all the 

 other parts of the body, and be left unprovided with a mem- 

 branous support. This membrane is analogous to the cellular 

 membrane; and if we admit that the filaments of the brain are 

 similar to the fibres in other parts of the nervous system, we 

 may consider the membrane which sustains and connects the 

 cerebral plexuses as their proper musclema. 



The pia mater, or vascular integument of the brain, is com- 

 posed of two layers ; the external of which passes over the 

 convolutions of the cerebrum and the gyri of the cerebellum, 

 and the internal is reflected between these forms, and gives all 

 their exterior surface an intimate covering. The blood-vessels 

 seen on the brain are inclosed between these layers, and are 

 conducted on the inner layer to the gubstance of the organ. The 

 inner portion of the pia mater is continuous with the membrane 

 of the substance of the brain, but becomes so delicate on enter- 

 ing the structure of the organ that it is readily detached from 

 the brain without apparently injuring the integrity of its sur- 

 faces. When the inner layer of the pia mater is obtained in 

 connexion with a portion of the vessels and membrane which 

 penetrate the brain, it has the appearance of tufts or shreds, 

 and as such has been described by Ruisch and Alkenus under 

 the name of tomentum cerebri. 



The musclema of the brain appears in the adult to be only 

 furnished with colourless vessels, except in those places where 

 red vessels are seen to pass into the substance of the organ ; 

 but in the foetus, the coloured substance of the convolutions 

 may be injected so as to appear quite red. This fact is con- 

 sistent with the structure of many other organs during foetal 

 life, which in that period of existence receives red injection, 

 yet only admits afterwards colourless fluids. The great degree 

 of vascularity in the fcetus is particularly remarkable in the eye, 

 the lining of the labyrinth of the ear, the periosteum, &c. 



The author has ascertained that the actual quantity of the 

 sentient substance existing in the brain and other parts of the 

 nervous system is extremely small. The bulk of these parts is 

 not materially diminished by removing their nervous matter, 

 provided their membranous structure be left behind ; and 

 whenever we meet with the sentient substance in connexion with 

 a highly attenuated membrane, as in the retina and in several 

 of the cerebral plexuses contained in the coloured matter of the 

 brain, it is absolutely invisible until it has imdergone some de- 

 gree of coagulation. It is, perhaps, not assuming too much 



